scripts: ci: test_plan: Specify ZEPHYR_BASE for list_boards

The commit c3620c8a changed the
`list_boards` script such that it no longer searches the boards from
the `ZEPHYR_BASE` path by default.

This commit updates the `test_plan` script to invoke the `list_boards`
script with the `ZEPHYR_BASE` as a search path.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>